1. Field of the Invention
The present invention generally relates to semiconductor devices, and more particularly, to a semiconductor device improved so that leak current can be reduced.
The present invention further relates to a method of manufacturing such a semiconductor device.
2. Description of the Background Art
FIG. 52 is a circuit diagram of a memory array of a conventional dynamic random access memory (DRAM). FIG. 53 is a diagram showing waveforms of array operation of the conventional DRAM.
Referring to these figures, a memory cell array of the conventional DRAM includes a word line (WL), a bit line (BL), a bit line isolation (BLI), a sense drive line (SN), a bit line precharge (VBL), a restore drive line (SP), a memory cell, an N channel-sense amplifier (Nch-SA), an equalizer, and a P channel-restore amplifier (Pch-RA).
Information is stored in the memory cell. A place in which reading/writing is to be carried out is designated by selection of a word line. Reading or writing is carried out by selection of a specified bit line.
FIG. 54 is a sectional view of the memory cell of the conventional DRAM. In the surface of a P-type semiconductor substrate 1, provided is a field oxide film 2 which isolates an active region 3 from other active regions with its end portion 2a surrounding active region 3. An active layer 4 which is an impurity diffusion layer is provided in the surface of active region 3. A gate electrode (word line) 5 is provided on active region 3. An interlayer insulating film 6 is provided on semiconductor substrate 1 so as to cover gate electrode 5. A contact hole 7 exposing the surface of active layer 4 is provided in interlayer insulating film 6. Contact hole 7 is filled with a storage node 8 which is a conductor electrically connected to the surface of active layer 4. The surface of storage node 8 is covered with a dielectric film 9. A cell plate electrode 10 is provided so as to cover storage node 8 with dielectric film 9 interposed therebetween. A bit line 12 is connected to an active layer 11.
In the memory cell of the DRAM, information is held by storing charge in dielectric film 9 sandwiched by storage node 8 and cell plate 10.
The stored charge leaks to decrease in accordance with time. When much of the stored charge leaks, accurate information cannot be read out, causing malfunction of the device.
Leakage current is mostly caused by defects in semiconductor substrate 1.
Description will be given of the leakage current in more detail.
FIG. 55 is a layout diagram of a DRAM. In FIG. 55, a contour 13 represents an end portion of a mask forming an active region, which will be described in more detail.
More specifically, referring to FIG. 57A, in order to form active region 3 on semiconductor substrate 1, a mask 14 of a silicon nitride film is formed on active region 3.
Referring to FIG. 57B, the surface of semiconductor substrate 1 is thermally oxidized using mask 14 to form field oxide film 2. At this time, an end portion of field oxide film 2 goes under mask 14 to form a bird's beak portion 22.
Referring to FIG. 57C, when mask 14 is removed, field oxide film 2 is formed which is divided into a body region 16 having an approximately constant thickness and a bird's beak region 15 having a thickness gradually decreased in the direction toward active region 3.
In FIG. 55, contour 13 represents an end portion 13 of mask 14. In other words, referring to FIGS. 55 and 57C, contour 13 represents the boundary between body region 16 and bird's beak region 15.
In FIG. 55, a contour 17 represents an opening portion of a resist mask for formation of a storage node contact hole, which will be described in more detail. More specifically, referring to FIG. 58A, in order to form a contact hole in interlayer insulating film 6, a resist mask 18 having a predetermined opening portion 17 is formed on interlayer insulating film 6. Referring to FIG. 58B, interlayer insulating film 6 is etched using resist mask 18 to form contact hole 7. Contour 17 represents an end portion of the above-described opening portion 17.
FIG. 56 is a sectional view of a memory cell taken in the same direction as the direction of the line 56--56 in FIG. 55. Field oxide film 2 is formed in the surface of semiconductor substrate 1. Active layer 4 is formed in the surface of active region 3. Interlayer insulating film 6 is formed on semiconductor substrate 1 so as to cover active layer 4. Contact hole 7 exposing the surface of active layer 4 is formed in interlayer insulating film 6. Contact hole 7 is filled with storage node 8. A depletion layer 20 extends from active layer 4.
As described above, leakage current is mostly caused by a defect 21 in semiconductor substrate 1. Referring to FIG. 57B, when field oxide film 2 is formed in the surface of semiconductor substrate 1, much stress is applied directly under bird's beak portion 22. Therefore, a lot of defects 21 are formed in the vicinity of the boundary between bird's beak portion 22 and semiconductor substrate 1. Metal ions generated in the manufacturing process of a semiconductor device are trapped in defects 21. Referring to FIG. 56, when depletion layer 20 extending from active layer 4 comes in contact with defects 21, a leakage current component other than leakage current in the direction opposite to the case of a normal PN junction is added, which in turn rapidly decreases charge stored in the memory cell. This is a problem of the conventional DRAM.